X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2000-06-02
2001-09-18
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S004000
Reexamination Certificate
active
06292527
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for operating a computed tomography (CT) device, of the type having an X-ray source at least the focus of which is, i.e. plane or curved at least two-dimensional displaceable around a system axis, a detector system, formed by a planar/array of detector elements that are arranged in rows that extend substantially transverse to the direction of the system axis and that also are arranged in columns, wherein the detector system picks up X-rays emitted from the X-ray source, and a support device for an examination subject, wherein the focus of the X-ray source is displaced around the system axis for scanning a volume of the examination subject for generating an X-ray shadow image of the examination subject is obtained.
2. Description of the Prior Art
The following method for operating the CT device is normally utilized for carrying out an examination by means of a CT device. An X-ray shadow image (topogram) is generated when the X-ray source does not rotate, with the examination subject being moved on the support device relative to the X-ray source and the detector system is moved in the direction of the system axis. The area of the examination subject to be acquired is defined, in the actual examination, in the direction of the system axis on the basis of the generated X-ray shadow image. The examination subject is positioned by moving the support device to the start point of the examination subject's defined area to be acquired. The actual examination is then conducted, usually in the form of a spiral scan.
This method is associated with a few basic disadvantages. The entire procedure is relatively long, which is undesired for reasons of efficiency and also for medical reasons, particularly with respect to emergency patients. Also, when X-ray shadow images from different projection directions (angles of view) are desired, for example “from front” and “from the side”, the patient is moved three times with the patient bed—namely twice for purposes of generating the shadow images and a third time for the spiral scan. There is the danger of the examination results being falsified due to movements of the patient between X-ray shadow images and spiral scan. The generation of one or more X-ray shadow images means an additional radiation exposure for the patient to be examined.
In a CT apparatus having a detector system with only one row of detector elements, these disadvantages can be theoretically avoided by the method known from European Application 0 531 993 and German OS 41 03 588, since merely a spiral scan is carried out and the data that are required for the reconstruction of the X-ray shadow image, from one or more projection directions, are extracted from the thereby-acquired data and are correspondingly processed. The resulting X-ray shadow image, or the resulting X-ray shadow images, keeping pace with tomograms that are reconstructed in parallel with the X-ray shadow image-reconstruction, are shown together on a display unit. However, in practical operation the image quality of the X-ray shadow image is insufficient, particularly when disadvantageous operating parameters are utilized, such as a slice thickness greater than 2 mm and/or a relation of bed advance per rotation to slice thickness (pitch) of greater than 1.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of the aforementioned type wherein the conditions are present to enable X-ray shadow images of high quality to be generated on the basis of a spiral scan that is carried out with disadvantageous operating parameters.
According to the invention, this object is achieved in a method for operating a computed tomography (CT) device, having an X-ray source at least the focus of which is displaceable around a system axis, a detector system, formed by a planar array of detector elements that are arranged in rows that extend substantially transverse to the direction of the system axis and that also are arranged in columns, wherein the detector system picks up the X-rays emitted by the X-ray source and a support device for an examination subject, wherein at least the focus of the X-ray source is displaced around the system axis for scanning a volume of the examination subject and wherein the generation of an X-ray shadow image of the examination subject ensues by means of the steps of implementing a volume scanning, extracting data that belongs to a desired projection direction from data supplied by a number of rows of the detector system in the volume scanning, and reconstructing the X-ray shadow image on the basis of the extracted data.
In a preferred embodiment of the invention, the volume scanning can be carried out in the form of a spiral scan; for this purpose, the support device, and the X-ray source and the detector system, are relatively displaced substantially in the direction of the system axis while rotating the X-ray source (or at least its focus) around the system axis.
Alternatively, the support device, and the detector system, and the X-ray source can assume a fixed position relative to one another in the direction of the system axis when the volume is scanned. In this case, the extent of the area of the examination subject covered by the volume scan in the direction of the system axis is determined by the size of the array of detector elements in the direction of the system axis, or is determined by the extent of the area of the array utilized for the volume scan in the direction of the system axis, if the entire array is not been utilized.
Regardless of the type of volume scan, the inventive method is based on the utilization of a CT device with a detector system, which does not have a single row of detector elements, but has a planar array with a number of rows of detector elements, and, from the data acquired in the course of the volume scan, preferably in the form of a spiral scan, the inventive method not only utilizes the data from one row of the detector system but utilizes data supplied by a number of rows of the detector system in order to reconstruct the X-ray shadow image.
As long as the collimated slice thicknesses do not substantially exceed 2 mm regarding the data supplied by the rows of the detector system, on which data the reconstruction of the X-ray shadow image is based, and as long as it is guaranteed—in the case of a volume scan in the form of a spiral scan—that the advance per rotation of the X-ray source focus does not exceed the total width of the detector rows supplying the data on which the reconstruction of the X-ray shadow image is based, the conditions for a high image quality of the X-ray shadow image exist.
In a version of the invention, at least one tomogram is reconstructed on the basis of the data acquired during the volume scan and the extraction of the data for the X-ray shadow image ensues prior to the reconstruction of the tomogram. It is thereby guaranteed that the processing of the data acquired during the volume scanning does not influence the image quality of the X-ray shadow image. The processing ensues in the course of the reconstruction of the tomogram. This is particularly important when, in an embodiment of the invention, the data of a number of rows of the detector system are combined for reconstructing the tomogram, for example, in order to be able to reconstruct tomograms of slices, whose thickness is larger than the width of a detector row. In this case, the combination of the data of a number of rows of the detector system does not affect the image quality of the X-ray shadow image.
Therefore, keeping pace—i.e. parallel to the volume scanning—X-ray shadow images of high quality and tomograms can be reconstructed and displayed by means of the inventive method.
If the data are picked up in fan geometry in a version of the invention the data are converted into parallel geometry for avoiding specific image artefacts in the X-ray shadow image that are caused by the fan geometry.
To improve the resolution in the direction of the
Kiknadze Irakli
Kim Robert H.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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